1. Field of the Invention:
This invention relates to lens surfacing apparatus and has particular reference to improvements in toric surface generators.
2. Discussion of the Prior Art:
A toric surface is a surface of compound curvature frequently used ophthalmically for the correction of astigmatism. By compound curvature it is meant that the radius of curvature in one meridian is different than the radius of curvature in a second orthogonal meridian. These curvatures are commonly referred to as the spherical and cylindrical components of the tore, the cylindrical component usually being of greatest power (i.e. shortest radius of curvature).
Because of the large commercial and practical importance of toric surfaces, a number of techniques have developed for their production. An early technique involved the use of preformed tools each having the shape of a particular toric curve desired on a lens, i.e. a mirror image of the desired lens surface shape. This preformed tool was abraded against the lens surface in conjunction with abrasive slurries in such a way that gradually the lens assumed the shape of the tool. Thus, reasonably accurate toric surfaces were produced. Because of rapid tool wear, however, and the vast inventory of preformed tools needed to satisfy the hundreds of combinations of the two meridianal radii of curvature encountered in the field, preformed tools have been largely replaced by a rotating cupped or ring tool.
The cupped or ring tool typically has an annular working edge which abrades the workpiece, be it glass or plastic. A toric surface is achieved by having the radius through which the ring tool is swung be substantially the same as one of the desired radii of lens surface curvatures. The second radius of curvature in a meridian at right angles to the first is achieved by a tilt of the ring tool so that the profile of the tool assumes approximately the curvature of the second radius. The universal nature of being able to modify independently both the radius of swing and the angle of tool tilt eliminates the need for large tool inventory. Nevertheless, in the process of using the angle of tilt to modify one effective tool cutting radius, an elliptical error is introduced so that the lens surface formed is not a true toroid. This elliptical error is in most cases significant. It requires subsequent surface grinding to eliminate.
A number of attempts have been made to overcome the problems associated with elliptical error. In one case, the elliptical error was minimized by moving the tool relative to the lens in a series of complex motions which necessitated correspondingly complex and expensive machinery not well suited for use in custom laboratory operations. More recent attempts used a grinding tool which was swung through one of the desired radii of curvature with its own radius of curvature being that of the second radius desired on the lens. This, however, necessitates a separate tool for each second radius of curvature and hence, still requires costly tool inventory. Such a need for large tool inventory can, however, be reduced by still using the aforementioned cupped or ring tool which is swung through one radius with the orthogonal tool profile assuming the curve along a second meridian which is at the same time modified with an oscillating motion of the lens relative to the tool. Although theorectically capable of producing desired surface curvatures, this scheme is cumbersome and difficult to implement and lacks the rigidity needed for optimum surfacing.
Examples of the above toric generating schemes and apparatuses can be found in U.S. Pat. Nos. 2,548,418; 2,633,675; 2,724,218; 3,117,396; 3,492,764 and 3,624,969.
With a view to overcoming the above and other shortcomings of the prior art, it is an object of this invention to simplify the manufacture of true toric or spherical surfaces and more particularly to avoid elliptical error in toric lens surfaces intended for the correction of astigmatism.
More specifically, an object of the invention is to accomplish the foregoing with surface generating apparatus capable of producing numerous combinations of spherical and cylindrical lens curvatures using a single fixed radius cutter.
Still another object is to provide for toric and spherical lens surfacing with minimal equipment complication and without need for special operating skills.
Other objects and advantages of the invention will become apparent from the following description.